Hydrostatic-transmission control system, especially for lift and other industrial vehicles

Abstract

A hydrostatic-transmission control system for fork-lift trucks and like industrial vehicles has at least one variable-displacement hydrostatic motor driving wheels on opposite sides of the vehicle and supplied by a pump having a control element for varying its displacement under the control of an operator. The prime mover, e.g. a gasoline engine or electric motor, drives the pump. The control system comprises means whereby, for low and average output speeds of the transmission consisting of a combination of the pump and motor, the control of the transmission ratio is a function only of the operator-set position of the aforementioned element whereas, for the highest output speeds, the ratio is controlled by both the position of this element and by a further parameter, usually the output speed of the prime mover.

I claim: 
     
       1. A drive comprising: a hydrostatic transmission including: a variable-displacement pump,   a setting member operatively connected to said pump for varying the displacement thereof,   at least one variable displacement hydraulic motor having a displacement controlling element and hydraulically connected to said pump so as to be driven thereby,   a motor-setting unit including a cylinder, a piston displaceable in said cylinder and connected to said element and a spring bearing upon said piston;     an internal-combustion engine connected to said pump for driving same;   a constant displacement auxiliary pump connected to said engine and driven thereby;   means forming a hydraulic-fluid path including a throttle leading from said auxiliary pump to a reservoir;   a line connecting said path between said auxiliary pump and said throttle with said cylinder; and   control means including said auxiliary pump, said fluid path, said line and means connected to said setting member for operating same to exclusively control the transmission ratio by varying the displacement of said variable-displacement pump at low to average output speeds of the transmission and for enabling said motor-setting unit to exclusively control said transmission ratio for high output speeds of the transmission.   
     
     
       2. A drive comprising: a hydrostatic transmission including: a variable-displacement pump,   a setting member operatively connected to said pump for exclusively varying the displacement thereof,   at least one variable displacement hydraulic motor having a displacement controlling element and hydraulically connected to said pump so as to be driven thereby,   a motor-setting unit including a cylinder, a piston displaceable in said cylinder and connected to said element and a spring bearing upon said piston;     an internal-combustion engine connected to said pump for driving same;   a constant displacement auxiliary pump connected to said engine and driven thereby;   means forming a hydraulic-fluid path including a throttle leading from said auxiliary pump to a reservoir;   a line connecting said path between said auxiliary pump and said throttle with said cylinder, said throttle being adjustable; and   means connecting said member to said throttle for varying the cross-section thereof only upon shifting of said variable-displacement pump to a maximum-displacement setting.